Since when is needing a firewall Windows specific? What are you talking about? Your post makes no sense to me. I've worked in both Unix/Windows environments and firewalls are used for both.

How is a cluster stand-alone as you imply?? A cluster needs a network and a network isn't stand-alone.

What's the big deal with having antivirus software installed? Some of them are system hogs (norton) but there are much better alternatives out there. Any environment needs to be set up and configured properly and securely, no OS is excluded from that.